South Korean Internet-savvy citizens, such as bloggers, Twitter users and online media reporters, who are invited by the Defense Ministry, visit the wreckage of a warship that the government claims was sunk by a North Korean torpedo in March, at the Second Fleet Command of Navy in Pyeongtaek, south of Seoul, South Korea, 08 Jul 2010.

A new study by U.S. researchers raises questions about the investigation into the sinking of a South Korean navy ship. International investigators blamed a North Korean torpedo, raising tensions on the Korean peninsula.

Researchers J.J. Suh and Seung-Hun Lee say the South Korean Joint Investigation Group made a weak case when it concluded that North Korea was responsible for sinking the Cheonan.

Speaking in Tokyo Friday, the two said the investigation was riddled with inconsistencies and cast "profound doubt" on the integrity of the investigation.  "The only conclusion one can draw on the basis of the evidence is that there was no outside explosion," Suh said. "The JIG completely failed to produce evidence that backs up its claims that there was an outside explosion."

Suh is an associate professor in international relations at Johns Hopkins University in the United States, where he runs the Korean studies program.

International investigators said in May that an external explosion caused the South Korean ship to sink last March, killing 46 sailors. The report said a North Korean-made torpedo caused the explosion.

Suh and Lee the cracked portion of the bottom of the ship does not show the signs of a large shock that are usually associated with outside explosions. They add that all the ship's internal parts remained intact and few fragments were recovered outside the ship.

"Almost all parts and fragments should've been recovered within about three to six meters within where the torpedo part was discovered," Lee says, "The fact that only the propeller and the propulsion part was discovered doesn't make any sense to me."

Lee is a professor of physics at the University of Virginia in the United States. Lee also points to a blue mark on a fragment of the torpedo to question the validity of the study. South Korean scientists say that part of the torpedo was marked "number one" in Korean, with a blue marker.

Suh and Lee say the writing would not have survived the intense heat of an explosion.  "This can not be taken as evidence. Because any Korean, North and South, can write this mark," Suh said. "Also, it does not make sense that this blue ink mark could survive so freshly when the paint all around was all burned at the explosion."

Both researchers say their findings do not prove that North Korea did not sink the Cheonan. But they say it is irresponsible for the South Korean government to reach its conclusions based on an inconclusive study.

They are calling for a new international investigation to re-examine the Cheonan's sinking. They also want the United Nations Security Council to pressure the South Korean government and request an "objective and scientific" report before the council deliberates on the incident.

Source: Voice of America